A similar multiple frequency tag is described by Kajfez et al. in WO 95/05647. The multiple frequency tag described by Kajfez et al. is comprised of a dielectric substrate. A first resonant circuit including a first inductor coil and having a first predetermined resonant frequency is located on the first surface of the substrate. A second resonant circuit including a second inductor coil and having a second predetermined resonant frequency which preferably is different from the first predetermined resonant frequency is located on the second surface of the substrate. The first inductor coil is positioned on the substrate to partially overlay the second inductor coil in a manner which minimizes the magnetic coupling between the first and second coils. The tag may be employed in any type of detection system including an electronic article security system for protecting articles of sale from theft. Tags of this kind are manufactured by lamination of aluminum foils on a dielectric substrate. This substrate is subsequently printed and etched to form the resonant coils and then coated with adhesive and a protective strippable cover. It is then cut to size and shape.
A corresponding manufacturing process for a similar resonant tag to be utilized for the same purpose has been disclosed by Imaichi et al. in EP 070318 B1.
The resonant circuits described by Kajfez et al., have an advantage in that they are difficult to deactivate and cannot be destroyed by magnets. However, the etching step for manufacture of these devices requires the precise printing of a non-etchable ink so that only the exposed areas of the aluminum foil are removed by etching. The uncertainties of this process combined with the unavoidable variations in dielectric thickness combine to make the process for production of these multiple frequency tags complex and undesirably expensive. Furthermore, this method of manufacture produces many tags with defective responses due to the resonant frequency being outside reasonable limits of variation.
Hultaker in U.S. Pat. No. 4,929,928 discloses the application of an ink comprising magnetizable particles to a theft protection device. However, the use of magnetizable particles as a means of theft prevention is easily obviated by a thief who has the means for destroying the magnetic properties of the device, for example, by applying a magnet.
Appalucci et al., in U.S. Pat. No. 5,142,270 and U.S. Pat. No. 5,241,299, describe a stabilized resonant tag circuit and deactivator for use in an electronic article surveillance system. The tag disclosed in these patents has a substantially planar dielectric substrate having conductors placed on either side where at least one of the conductors includes an inductor. The tag is stabilized by a flexible, substantially planar, tear-resistant, polymeric film adhered to and covering one of the conductors and the substrate. The film provides a vapor barrier which minimizes the effects of body detuning on the circuit and promotes the secured integrity of the tag. The tag may further comprise a deactivator for deactivating the tag in response to an electromagnetic field of sufficient energy to destroy the resonant properties of the circuit, the deactivator being an indented portion of at least one of the conductors such that the conductors are closer to each other at the indented portion than at the remainder of the conductor. The conductors of this device are made of a metallic conductor material such as aluminum foil and prepared using an extrusion coating process not described. The polymeric film which adheres to the conductors and the substrate provide mechanical stability, while the covering polymeric film provides a thin layer impervious to water vapor or other contaminants which may alter the resonating frequency.
In the present invention, radio frequency resonant tags comprising polymeric materials in the entirety are provided which are manufactured via a quick and reliable process with limited variability to be deactivated only when exposed to a strong electromagnetic field at a selected resonant frequency.